The present invention relates to a packet configuring method and a packet receiver. Particularly, the present invention relates to a packet configuring method and a packet receiver, each for configuring a packet that contains training sequences in an asynchronous packet communication mode.
FIG. 7 is a diagram illustrating a down-link control channel frame used for the conventional digital automobile telephone, described ETSI/GSM, xe2x80x9cRecommendation GSM 05.02 Multiplexing and Multiple Access on the Radio Pathxe2x80x9d, version 3.3.0, 15, Apr., 1989. FIG. 7 shows an example of a 10-channel configuration being the head of a frame. This frame is formed of a frequency connection channel (FCCH) 1000, a synchronization channel (SCH) 1001 and a broadcast channel (BCCH) 1002. The slot forming SCH 1001 or BCCH 1002 is formed of the training portion 1011 and the data portions 1010 sandwiched by the training portion 1011. The frequency connection channel FCCH 1000 is formed of a sine wave signal with a single frequency.
The mobile station that receives the control channel operates as follows: First, the mobile station receives FCCH 1000 and then corrects a variation in frequency (frequency offset) between a transmitter and a receiver. Then, the mobile station demodulates the synchronization channel SCH 1001 and the broadcast channel BCCH 1002.
SCH 1001 and BCCH 1002 are demodulated as follows: First, a channel impulse response is obtained using the training portion 1011. SCH 1001 and BCCH 1002 are demodulated by setting the reception parameter for the receiver based on the resultant channel impulse response. That is, the training portion 1011 is used for the initializing of the receiver.
According to the conventional art described above, frequency offset compensation and channel impulse response estimation necessary for signal reception are differently obtained.
In the automobile telephone system where communications are not always established through a base station but either communications via the base station or direct communications between terminals are established, for example, in local area networks (LANs), there is the possibility that different signal transmission sources are used for respective packets. This requires the receiver to execute frequency offset compensation and channel impulse response estimation every packet reception. In such a case, it may be considered, as shown in FIG. 8, that both the sequences 1020 for frequency offset estimation and the sequences 1021 for channel impulse response estimation are contained in the training sequences.
There is the method where the receiver monitors, for example, the reception power and the packet transmission to detect a transmitted packet in an asynchronous packet transmission and detects when the reception power exceeds a predetermined threshold value. In this case, the head of a packet cannot be already received accurately due to influences of noises or radio transmission path. Hence, this method has the disadvantage in that the boundary between the sequence for frequency offset estimation and the sequence for channel impulse response estimation may not be recognized.
Moreover, the method has the disadvantage in that the length of a training sequence is prolonged using the sequence for frequency offset estimation and the sequence for channel impulse response estimation, whereby the transmission efficiency is degraded.
The present invention is made to solve the above-mentioned problems.
Moreover, the objective of the invention is to provide a packet configuration method that correctly estimates an estimate frequency offset and a channel impulse and then demodulates a received packet even when a transmitted packet is erroneously detected in timing in the communication mode where packets are asynchronously transmitted.
Another objective of the present invention is to provide a packet receiver that correctly estimates an estimate frequency offset and a channel impulse response and then demodulates a received packet even when a transmitted packet is erroneously detected in timing in the communication mode where packets are asynchronously transmitted.
The objective of the present invention is achieved by a method configuring packets, the packets each having a training portion and a data portion to set a receiver, comprising the step of forming the training portion by serially connecting K sequences (where K is an integer of 2 or more), each of the K sequences being formed of N symbols (where N is an integer of 2 or more).
According to the present invention, a packet receiver receives packets each which is formed of a training portion and a data portion to initialize of the receiver. The packet receiver comprises frequency-offset estimation means for estimating a frequency offset based on a received packet, frequency-offset compensation means for compensating a frequency offset contained in the received packet based on the frequency offset estimation value, and channel impulse response estimation means for estimating an impulse response of a channel based on an output of which the frequency offset is compensated.
That is, the same sequences, each formed of N symbols, are repeatedly used in a communication mode where packets are asynchronously transmitted. Thus, the frequency offset can be estimated by detecting the phase difference between a signal received before NT and a currently-received signal.
In a packet formed of a training portion and a data portion to initialize a packet receiver according to the present configuring method, the training portion has K sequences chained, each being formed of N symbols, to estimate a frequency offset and a channel impulse response. The auto-correlation function of a sequence formed of N symbols is in an impulse state.
Moreover, the packet receiver comprises a frequency offset estimation circuit for estimating a frequency offset of a received signal and then outputting a frequency offset estimation value, a frequency offset compensation circuit for receiving the frequency offset estimation value and the received signal and compensating a frequency offset contained in the received signal based on a frequency offset estimation value, and a channel impulse response estimation circuit for receiving an output from the frequency offset compensation circuit, estimating a channel impulse response, and then outputting the channel impulse response estimation value after inputting a frequency offset estimation completion pulse.
The frequency offset estimation means comprises a delay circuit for receiving a received signal and delaying the received packet by a transmission period of time corresponding to N-symbols; a phase difference detection circuit for detecting a phase difference between an output of the delay circuit and the received packet; an integrator for integrating a detection output of the phase difference detection circuit over a transmission period of time of a sequence of M symbols; and a divider circuit for dividing an output of the integrator by a product of N and M.
According to the present invention, the sequence for estimation of a frequency offset and the sequence for estimation an impulse response are not separated from each other and are defined as repetition of the same sequences. The phase difference between the i-th symbols in each sequence is detected using this configuration, so that the frequency offset can be estimated. This feature allows the frequency offset to be estimated correctly even when a packet is detected with an erroneous timing.
Furthermore, where the auto-correlation function in an impulse state is used as the same sequence, a channel impulse response can be estimated by the simple operation that the receiver examines the correlation between the sequence and a received training sequence.
According to the present invention, since the frequency offset estimation and the channel impulse response estimation can be performed using the same sequences, the frequency offset estimation sequence and the channel impulse response estimation sequence are equivalently overlapped. This configuration can reduce the length of the training sequence.